It has been long known that paint coatings could be made electrically conductive by the inclusion therein of particulates comprised of matter having substantial electrical conductivity properties. A number of paints are known incorporating electrically conductive metal particles or particles of carbon therein which can function to conduct, for example static electricity, accruing in a structure which the paint is protecting.
More recently, with the advent of elaborate solid state circuitry devices the requirements for performance characteristics of conductive coatings have become more stringent. These solid state devices include electronic memories susceptible to so-called EMI (electromagnetic wave interference) or RFI (radio frequency interference). EMI and RFI have demonstrated a capability for de-programming memories and the potential for ruining circuitry thus rendering complicated electronic circuitry devices useless. In the past, EMI or RFI sensitivity was not of particular concern to the electronics industry as electronics devices typically were housed in metallic housings which functioned also to shield electronic devices from EMI or RFI interference.
More recently, plastics have found increasing use in forming cabinetry for containing electronic components and, typically, such plastics are sufficiently electrically non-conductive whereby, unlike metal housings, no significant shielding from RFI or EMI electromagnetic radiation is achieved.
Electrically conductive paints have therefore achieved some acceptance in imparting electromagnetic radiation protection to electronic circuitry contained in plastic cabinetry. Such conductive paint coatings are typically applied to interior surfaces of cabinetry encapsulating electronic components and then are grounded in suitable or conventional fashion to dispose properly of electrical energy generated by impinging electromagnetic radiation.
Such paint coatings configured for protecting electronic devices from EMI and RFI electromagnetic radiation must provide substantial conductivity to be effective. While various entities within the electronics industry have varying standards for measuring the conductivity performance for such coatings, it would appear that a generally universal performance standard for EMI/RFI cabinetry protection of about 1 .OMEGA./square after aging would appear to be emerging.
It is known to formulate coatings including electrically conductive additives such as carbon or metals in particulate form where the coatings have a solvent base. More recently, coatings have been formulated having a water base and including electrically conductive particulates of, for example, carbon or metal particles.
A number of factors influence the acceptability of a particular coating for use on a paint spraying line such as may be associated with the manufacture of electronics components cabinetry having an applied EMI/RFI paint coating upon inner surfaces of the cabinetry. Desirably, such coatings: (i) should be air dryable rather than requiring oven drying; (ii) should be substantially free from a tendency for rapid settling of any included particulates so that constant or frequent agitation of the coating during application on the spraying line is not necessary and so that upon shutdown of a spray line for an extended period such as over a weekend, settling in spray hoses, nozzles and paint containers will not cause serious spray line difficulties; (iii) should have good properties of adhesion to the plastic cabinetry; (iv) should have outstanding storage stability especially with reference to settling of particulate therein; and in addition (v) should be substantially resistant to the effects of abrasion, humidity, and temperature cycling.
Particularly, solvent based conductive paints traditionally have included a significant quantity of entrained solvents which evaporate upon pain drying. The size and cost of physical equipment necessary to cope adequately with recovery of such solvents on a commercial scale, for reasons such as the prevention of air pollution or solvent reuse, can detract significantly from the desirability of using solvent based paint coatings in effecting EMI/RFI protection for electronics hardware cabinetry.
Water based coatings, often also known as latex paints, tend to evolve relatively small quantities of solvent upon drying but are more prone to difficulties with: (i) adhesion, particularly to the surfaces of certain plastics such as NORYL.RTM., or LEXAN.RTM. (General Electric Co.), CALIBRE.RTM. or STYRON.RTM. (Dow), CYCOLAC.RTM. (Borg Warner), or GEON.RTM. (BFGoodrich); ii) settling problems while in use on a spray line; and (iii) a final conductivity of the coating being undesirably greater than the preferred maximum of 1 .OMEGA./square. Particularly, those water based paints employing metal flakes and most particularly nickel flake particles as the conductive particulate within the paint coating have tended to demonstrate undesirable settling characteristics and, to a considerable extent, substantially undesirable electrical conductance properties.
A water based, electrically conductive, air drying water based paint having settling characteristics suitable for use in spray lines wherein shutdowns of 72 hours or more may be tolerated without detrimental effect upon the spray equipment would have substantial application in the electronics industry particularly where the coating resulting from application of such a paint to a plastic electronics cabinet component is a coating having an electrical conductance parameter of 1 .OMEGA./square or less.